1. Field of the Invention
Our invention relates to an environmentally friendly burner assembly and method of using the burner assembly to combust mixtures of fuel and air and fuel, air and recirculated combustion gases. More specifically, our improved burner design uses rapid mixing of primary fuel and air in a venturi system to reduce the combustion temperature and minimize flame volume. Our burner may also recirculates combustion products to minimize the formation of NOx, which is a precursor for air borne pollutants.
2. The Prior Art
Industrial gas burners are designed to generate heat and produce high combustion temperatures, typically in the range of from 2500 to 3000° F. At such temperatures, thermal nitrogen oxides (NOx) can form as gaseous byproducts of the combustion of air and the gas used as the fuel in the burners. These NOx byproducts are a major source of air pollution and governmental authorities have instituted strict environmental regulations limiting the amount of NOx gases that can be emitted into the atmosphere. The art has recognized that reducing the peak flame temperature of industrial burners can minimize NOx formation. Increasing the air/fuel ratio reduces the peak flame temperature. Also, as taught in U.S. Pat. No. 5,073,105, lower flame temperatures may be achieved by recirculating a small portion of exhaust gases (also known as furnace or flue gases) into the combustion zone to mix with the fuel and combustion air. Specifically, the recirculated furnace gases are mixed with fuel gas followed by mixing with the combustion air before combustion. In our invention, the primary fuel mixes with the combustion air then the recirculated flue gas mixes with that mixture. The secondary fuel mixes with recirculated flue gas before it mixes with the air from the interior of the tile. U.S. Pat. Nos. 6,007,325 and 5,984,665 describe a burner design that-has three flame regions, where the first region is formed using a pre-mix burner tip to combust a lean fuel-air mixture. In U.S. Patent Application Nos. US 2002/0064740; US 200110034001; US 2002/0015930; and US 2002/0064740 a number of venturi type premix designs are disclosed, each with a specific tip design. The venturi tip designs restrict and disrupt the flow of the premix exiting the venturi. In addition to the pre-mix burner tip, these designs also use recirculated furnace gases. Previous burner designs have used staging of the air, staging of the fuel, a combination of air and fuel staging, and internal combustion product recirculation to limit the formation of NOx in the combustion process. All of these methods inhibit the mixing of the fuel and air, which results in larger flame volumes and in some cases poor stability. Other known burner designs use the rapid mixing of the primary fuel and air but suffer the problem of instability and “flashback” of the flame into the primary fuel and air mixer assembly.
Our invention solves all of these problems because it uses rapid mixing of the primary fuel and air in a venturi system to minimize the flame volume and eliminates flash back. A side benefit is the reduced size of the burner that permits it to fit existing openings for conventional burners. Our design generates low amounts of NOx gases, typically in amounts of less than 10 ppmv.
Accordingly, an object of our invention is to provide a burner design and method of using the burner for heating industrial furnaces, boilers, incinerators and other commercial equipment while generating low levels of NOx emissions.